Rechargeable lithium-ion batteries are extensively used in portable electronic devices such as mobile telephones and laptops, and are finding increasing application in electric or hybrid electric vehicles. However, there is an ongoing need to provide batteries that store more energy per unit mass and/or per unit volume.
The structure of a conventional lithium-ion rechargeable battery cell is shown in FIG. 1. The battery cell includes a single cell but may also include more than one cell. Batteries of other metal ions are also known, for example sodium ion and magnesium ion batteries, and have essentially the same cell structure.
The battery cell comprises a current collector for the anode 10, for example copper, and a current collector for the cathode 12, for example aluminium, which are both externally connectable to a load or to a recharging source as appropriate. A composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12 (for the avoidance of any doubt, the terms “anode” and “cathode” as used herein are used in the sense that the battery is placed across a load—in this sense the negative electrode is referred to as the anode and the positive electrode is referred to as the cathode).
The cathode comprises a material capable of releasing and reabsorbing lithium ions for example a lithium-based metal oxide or phosphate, LiCoO2, LiNi0.8Co0.15Al0.05O2, LiMnxNixCo1−2xO2 or LiFePO4.
A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16. A liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16. The polymer electrolyte material can be a solid polymer electrolyte or a gel-type polymer electrolyte and can incorporate a separator.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide cathode layer 16 via the electrolyte into the anode layer 14. In the case of a graphite-based anode layer, the lithium reacts with the graphite to create the compound, LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of 372 mAh/g. (“active material” or “electroactive material” as used herein means a material which is able to insert into its structure, and release therefrom, metal ions such as lithium, sodium, potassium, calcium or magnesium during the respective charging phase and discharging phase of a battery. Preferably the material is able to insert and release lithium.)
The use of a silicon-based active anode material is also known in the art. Silicon has a substantially higher maximum capacity than graphite. However, unlike active graphite which remains substantially unchanged during insertion and release of metal ions, the process of insertion of metal ions into silicon results in substantial structural changes, accompanied by substantial expansion. For example, insertion of lithium ions into silicon results in formation of a Si—Li alloy. The effect of Li ion insertion on the anode material is described in, for example, “Insertion Electrode Materials for Rechargeable Lithium Batteries”, Winter et al, Adv. Mater. 1988, 10, No. 10, pages 725-763.
WO2009/010758 discloses the etching of silicon powder in order to make silicon material for use in lithium ion batteries. The resulting etched particles contain pillars on their surface. The pillared particles may be fabricated by etching a particle having an initial size of 10 to 1000 microns.
WO 2010/040985 disclosed a method of etching particles having a mean particle diameter in the range of 5-500 microns.
U.S. Pat. No. 7,402,829 discloses etching of a silicon substrate to form an array of silicon pillars extending from the silicon substrate.
JP 2004281317 discloses growth of silicon nanowires by vapour deposition on a substrate for use in a lithium ion battery anode.
US 2010/0285358 discloses silicon nanowires grown on a substrate for use in a lithium ion battery.
US 2010/0297502 discloses silicon nanowires grown on carbon particles for use in a lithium ion battery.
US 2008/0261112 discloses a network of entangled silicon nanowires connecting silicon particles for use in a lithium ion battery anode.
WO 2011/117436 discloses a carbon nanofibre including a plurality of crystalline whiskers extending from the surface of the carbon nanofibre.